1. Field of the Invention
The present invention relates to a display panel. More particularly, the present invention relates to a display panel of a liquid crystal display device.
2. Description of Related Art
Conventionally, the application of liquid crystal material as liquid crystal display (LCD) has been broadly used in many display fields. The generation of LCD twisted nematic (TN), super twisted nematic (STN) LCD and double layer STN-LCD has evolved to the use of thin film transistor (TFT) as pixel switching elements. The conventional LCD has advantages of thin thickness, light-weight, low power consuming and radiation free compared to the conventional cathode ray tube (CRT) display. However, the conventional LCD at least has the problem of dependence of brightness and contrast ratio on view angle, and the problem of gray scale inversion.
FIG. 1A is a schematic explosive view illustrating a structure of a conventional LCD panel. Referring to FIG. 1A, LCD panel 100 generally includes an upper substrate 102, a bottom substrate 104, liquid crystal molecules 106 disposed between the substrates 102 and 104, polarizers 108 and 110 disposed above the substrate 102 and under the substrate 104 respectively, and a light source 112. In general, the bottom surface of the substrate 102 and the upper surface of the substrate 104 that are adjacent to the liquid crystal molecules 106 have to be rubbed to form an alignment surface for aligning the liquid crystal molecules 106. The directions D1 and D2 are perpendicular to each other, and they represent the rubbing directions of the bottom surface of the transparent substrate 102 and the upper surface of the substrate 104. As shown in FIG. 1A, the liquid crystal molecules 106 adjacent to the bottom surface of the substrate 102 and the upper surface of the substrate 104 are aligned parallel to the directions D1 and D2 respectively. Between the substrate 102 and 104, the liquid crystal molecules 106 are aligned in directions that rotate through 90° from the bottom surface of the substrate 102 to the upper surface of the substrate 104. In addition, the directions D3 and D4 of the absorption axis of the polarizers 108 and 110 are parallel to the directions D1 and D2 respectively.
In FIG. 1A, there is no voltage difference between the substrates 102 and 104. Therefore, when an unpolarized light source 112 passes through the polarizer 110, the polarization direction of the transmitted light is perpendicular to the direction D4. Then, the polarization direction of the transmitted light will be rotated by the liquid crystal molecules 106 and will be perpendicular to the direction D1 finally. Therefore, the transmitted light may pass through the polarizer 108 since the direction D3 of the absorption axis of the polarizers 108 is parallel to the direction D1.
FIG. 1B is a schematic explosive view illustrating an operation of a conventional LCD panel. In FIG. 1B, a voltage V is applied between the substrates 102 and 104, and thus the liquid crystal molecules 106 will be aligned parallel to the direction of the electric field generated by the voltage V. Therefore, when an unpolarized light source 112 passes through the polarizer 110, the polarization direction of the transmitted light is perpendicular to the direction D4. However, the polarization direction of the transmitted light is not rotated by the liquid crystal molecules 106 and will be still perpendicular to the direction D4 finally. Therefore, the transmission light can not pass through the polarizer 108 since the directions D3 of the absorption axis of the polarizers 108 is perpendicular to the direction D4. Accordingly, the brightness of the LCD panel 100 is dependent on the voltage V.
However, as shown in FIG. 1B, it should be noted that, only a middle portion of liquid crystal molecules 106 is aligned parallel to the direction of the electric field, the liquid crystal molecules 106 that near the bottom surface of the substrate 102 and the upper surface of the substrate 104 are still influenced by the rubbing directions D1 and D2 respectively. Therefore, a light leakage may be generated and the performance such as the contrast ratio of the LCD panel 100 may be reduced.
In order to improve the problem of the conventional LCD shown in FIGS. 1A and 1B, an optical compensation film is developed. FIG. 2 is a schematic explosive view illustrating a structure of another conventional LCD. Referring to FIG. 2, except for the basic structure of the LCD panel 100, the LCD panel 200 further includes optical compensation films 212/214 disposed between the substrate 102/104 and the polarizer 108/110 respectively. The direction D5 and D6 of the absorption axis of the optical compensation films 212 and 214 are disposed the same as the directions D3 and D4. The optical compensation films 212 and 214 are provided for compensating the difference of the reflective index of the liquid crystal molecules 106 as shown in FIG. 1B. Therefore, the problem of light leakage is prevented and the performance such as the contrast ratio of the LCD panel 200 is enhanced.
However, the LCD panel 200 shown in FIG. 2 has the following disadvantages. FIG. 3A is a plot of a contrast ratio versus a viewing angle of the LCD shown in FIG. 2. FIG. 3B is a schematic diagram illustrating a definition of the coordinate system for the viewing angle. It is noted that, the viewing angle of the plot of FIG. 3A is defined in FIG. 3B, wherein the direction 312 is the observation direction of the observer, the azimuthal coordinate is defined as the angle between the X-axis (0 degree) and the projection of the direction 312 on the surface of the substrate 102 of the LCD panel 200, and the radial coordinate is defined as the angle between the direction 312 and the Z-axis (i.e., the normal of the surface of the substrate 102 of the LCD panel 200.) The X-axis and the Z-axis are also illustrated in FIG. 2. The diagram shown in FIG. 3A represents the contrast ratio of the LCD panel 200 versus the viewing angle of the direction 312, wherein the contrast ratio is defined as the ratio of the brightness of pixel in bright (without voltage between the substrates 102 and 104) to the brightness of the pixel in dark (with voltage between the substrates 102 and 104). It should be noted that, the contrast ratio of the hatched area shown in FIG. 3 is larger that 10, however, the contrast ratio of the area having an azimuthal coordinate is close to 270° is less than 10.
FIG. 4 is a plot illustrating a gray scale inversion phenomenon of the LCD shown in FIG. 2. As shown FIG. 4, it is noted that as the azimuthal coordinate is close to 90° or 270°, the gray scale inversion phenomenon is very serious since the gray scale of the marked areas 404, 404 shown in FIG. 4 varies many times.
FIG. 5 is a diagram illustrating an observer and the LCD shown in FIG. 2. Referring to FIG. 5, the image displayed by the LCD 200 shown in FIG. 2 has a polarization direction D7 perpendicular to the direction D3. However, when the observer wears a pair of sunglasses 502, only a portion of the image having a polarization direction perpendicular to D8 may be observed since the conventional sunglasses 502 generally has an absorption axis with a direction D8 for filtering the sunlight. Therefore, the observed brightness of the image displayed by the LCD 200 may be reduced drastically.
Accordingly, an LCD for improving the low contrast ratio as the azimuthal coordinate is close to 270°, reducing the gray scale inversion problem and avoiding the reduction of brightness when a user views the LCD through a pair of polarized sunglasses is highly desirable.